Prenatal diagnosis began 30 years ago (See e.g., Williamson Bob, Towards Non-invasive Prenatal Diagnosis, Nature Genetics, 14:239–240 (1996)). Now, prenatal diagnosis has become a very promising field. Currently, fetal cells are obtained by using amniocentesis or chorionic villus sampling (CVS). Amniocentesis is the removal of amniotic fluid via a needle inserted through the maternal abdomen into the uterus and amniotic sac. CVS is performed during weeks 10–11 of pregnancy, and is performed either transabdominally or transcervically, depending on where the placenta is located; if it is on the front, a transabdominal approach can be used. CVS involves inserting a needle (abdominally) or a catheter (cervically) into the substance of the placenta but keeping it from reaching the amniotic sac. Then suction is applied with a syringe, and about 10–15 milligrams of tissue are aspirated into the syringe. The tissue is manually cleaned of maternal uterine tissue and then grown in culture. A karyotype is made in the same way as amniocentesis. Amniocentesis and chorionic villus sampling each increases the frequency of fetal loss. For amniocentesis, the possibility is about 0.5%, while for CVS, it is about 1.5% (U.S. Pat. No. 5,948,278; and Holzgreve et al., Fetal Cells In the Maternal Circulation, Journal of Reproductive Medicine, 37(5):410–418 (1992)). Therefore, they are offered mostly to women who have reached the age of 35 years, for whom the risk of bearing a child with an abnormal karyotype is comparable to the procedure-related risk.
Because of the uncertainties of the procedure-induced risks of amniocentesis and CVS, there is considerable interest in developing noninvasive methods for the information of gestating fetus. The existence of fetal cells in the maternal circulation has been the topic of considerable research and testing over many years. It is now understood that there are three principal types of fetal cells: lymphocytes, trophoblasts and nucleated fetal erythrocytes. (Simpson and Elias, Isolating Fetal Cells in Maternal Circulation for Prenatal Diagnosis, Prenatal Diagnosis, 14:1229–1242 (1994); Cheung et al., Prenatal Diagnosis of Sickle Cell Anaemia and Thalassaemia by Analysis of Fetal Cells in Maternal Blood, Nature Genetics, 14:264–268 (1996); Bianchi et al., Isolation of Fetal DNA from Nucleated Erythrocytes in Maternal Blood, Proc. Natl. Acad. Sci. USA, 86:3279–3283 (1990); and U.S. Pat. No. 5,641,628). Various proposals have been made for the isolation or enrichment of one of these cell types from a maternal blood sample, and it has been proposed to use these isolated or enriched cells for testing for chromosomal abnormalities. Trophoblasts are the largest cells of the three types of cells. But they have not found widespread application in separation studies because they are degraded in the maternal lung when they first enter the maternal circulation. Because fetal lymphocytes can survive quite a while in maternal blood, false diagnosis is possible due to carry over of lymphocytes from previous fetus. Nucleated red blood cells (NRBC) are the most common cells in fetal blood during early pregnancy. The separation methods that have been tested so far are fluorescence-activated cell sorting (FACS), magnetic-activated cell sorting (MACS), charge flow separation (CFS) and density gradient centrifuge. All of these methods result in the enrichment of fetal cells from a large population of maternal cells. They do not enable recovery of pure populations of fetal cells (Cheung et al., Nature Genetics, 14:264–268 (1996)).
There are two reasons for the difficulty. First, there are very few fetal NRBC in maternal blood although the number is high comparing to fetal trophoblasts and fetal lymphocytes. In maternal blood, the ratio between nucleated cells and fetal NRBC is 4.65×106˜6×106. About 7˜22 fetal NRBC can be obtained from 20 ml maternal blood by MACS (Cheung et al., Nature Genetics, 14:264–268 (1996)). Second, there is little difference between fetal NRBC and maternal cells. For fetal NRBC and maternal NRBC, the only difference between them is that there are specific hemoglobin γ and hemoglobin ζ in fetal NRBC.
Various techniques in a variety of fields, such as biology, chemistry and clinical diagnosis have been applied to cell separation. With these techniques, differences between cell types are exploited to isolate a particular type of cells. These differences include cell surface properties, and physical and functional difference between cell populations. In some cases, the difference between cell types is very trivial and it is very hard to separate them by current available techniques.
There exists a need in the art for a new process and device for cell separation and isolation. This invention address this and other related needs in the art.